Gas Diffusion: HCl & NH3 Reaction Time

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erjkism
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i did an experiment involving gas diffusion through a glass tube. HCl was on one side of the tube, while NH3 was on the other side. it took about 5 minutes for the gases to meet in the middle of the tube and form a precipitate.

but the mean velociries of the HCL and NH3 gas molecules at 25 degrees celsius are around 500 m/s. why did it take so long for the molecules to meet and react?
 

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Most of the time the 500m/s HCL molecule is bouncing off other HCL molecules.
Imagine a crowd where everyone is running around in circles bumbing into each other - the crowd as a whole only moves very slowly.
 
But would not the size of the molecules also effect the transit time. Not being a wise guy, just trying to learn as well. My thinking is the NH3 would be larger, so it would be like cramming a bunch of stuff in a small hole.
 
actually the HCl molecules are larger, and they ended up going quite a bit slower than the lighter NH3 molecules. It goes along with grahams diffusion rate equations